Some definitions of interest.
IdLnk	Def IdLnk == IdId
	Thm* IdLnk  Type
Id	Def Id == Atom
	Thm* Id  Type
ldst	Def destination(l) == 1of(2of(l))
	Thm* l:IdLnk. destination(l)  Id
lnk-inv	Def lnk-inv(l) == <1of(2of(l)),1of(l),2of(2of(l))>
lsrc	Def source(l) == 1of(l)
	Thm* l:IdLnk. source(l)  Id

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